A torque converter is typically placed between an internal combustion engine and an automatic transmission device and operative to transmit torque therebetween, using an impeller and a turbine device in a fluidic medium. A torque converter clutch typically comprises a pressurized fluid-actuated friction device engageable to mechanically couple the impeller, receiving input from the engine, and the turbine, having an output to the transmission. In a typical application, the clutch can be fully released, actuated in a slip mode, and fully engaged, i.e. locked. When the clutch is fully released, there is unrestrained slippage between the impeller and the turbine, and torque is transmitted therebetween based upon the flow of hydraulic fluid between the impeller and the turbine. When the clutch is actuated in the slip mode, torque is transmitted between the impeller and the turbine through the flow of hydraulic fluid therebetween and controlling pressure of hydraulic fluid to the actuated clutch, and typically there is a difference in rotational speeds between the impeller and the turbine, i.e., a relative speed. When the clutch is fully released, or actuated in the slip mode, torque perturbations between the engine and the transmission resulting from either engine operation or driveline dynamics are absorbed in the fluid of the torque converter.
When the clutch is fully engaged, the rotational speeds of the impeller and the turbine are the same, and torque is transmitted between the impeller and the turbine through the actuated torque converter clutch. When the torque converter clutch is fully engaged, a range of engine torque perturbations or torsionals, typically in the range of 2 to 6 Hz, are passed directly through the clutch to the vehicle drivetrain, producing pulsations therein when not properly damped. Other torsionals, typically those above about 20 Hz, are absorbed in a torsional damper device, which is an element of the torque converter. Thus, the action of completely locking the torque converter clutch is often restricted to specified vehicle operating conditions to minimize the effects on noise, vibration and harshness (NVH). As a result, potential efficiency gains afforded by fully engaging the torque converter clutch are only realized over a portion of the range of vehicle operation.
To overcome the disadvantages of torque converter clutch engagement, it has been proposed to operate the clutch in a slipping mode wherein a predetermined amount of slippage between the torque converter impeller and turbine is permitted for regulating the torque capacity of the clutch. In any such system, the objective is to isolate engine torque perturbations in the torque converter while passing steady state engine torque at a slip rate that provides improved torque converter efficiency, leading to improved fuel economy. Previous control systems proposed to manage clutch slippage have been disclosed, for example, in U.S. Pat. No. 4,582,185 to Grimes et al., issued Apr. 15, 1986, and U.S. Pat. No. 5,484,354, to Vukovich, et al., issued Jan. 16, 1996, each which is assigned to the assignee of the present invention. The advent of cylinder deactivation systems has further emphasized a need to effectively manage operation and control of a torque converter clutch in a modern powertrain system.
There is a need to expand range of usage of the torque converter clutch in order to gain efficiency benefits therefrom without adversely affecting driveability.